The invention relates to thermoplastic containers such as bottles, and more particularly, to the lubrication of chain belts which contact such containers during processing to prevent crazing of such containers.
Blow molded plastic bottles, such as those made from polyethylene terephthalate, have largely replaced heavier glass bottles previously used for carbonated beverages and the like. One disadvantage associated with plastic bottles is the extremely thin wall construction of the body of the bottle. The bottles are inherently weak which prevents them from being returned to the bottler and refilled. One attempt to overcome this disadvantage has been manufacturing plastic bottles of a one-piece construction with thicker body walls which make the bottles stronger, enabling them to be returned to the bottler for refilling. However, such bottles have a serious stress cracking problem, i.e. the development of fine cracks which are the result of the release of stresses introduced into the bottle during the molding process, particularly in the base area of the bottles which is complexly configured so as to both make the bottle free standing and to relieve internal stresses from pressurization. In addition, it has been found that stress introduced into the bottles in the blow molding process is accentuated when the bottles are recycled, because the hot caustic wash, pressurizing the bottles and abrading the base area of the bottles on a conveyor cause hazing or stress cracking on the exterior surfaces of the bottle. If severe enough, the stress cracking can result in loss of pressure and premature rupturing of the bottles.
Copending U.S. application Ser. No. 07/691,660 filed Apr. 26, 1991, commonly assigned, discloses a pressurizable thermoplastic container with a polyurethane layer on its exterior surface. The container can be subjected to multiple hot caustic washings and refillings with carbonated beverages with minimal, if any, stress cracking. Such containers can be prepared by applying, typically by a coating process, a continuous film of a polyurethane over the exterior surface of the thermoplastic container.
However, both coated and uncoated thermoplastic bottles are subject to crazing as a result of contact with chain belts during their conveyance through the various phases of recycling. Chain belt lubricants are sought to minimize this damage.